1. Field of the Invention
The present invention relates to a temperature control device for a fusing device in an image forming apparatus, and more particularly, it relates to a temperature control device for a fusing device in an image forming apparatus where the fusing device thermally fuses a toner image on a sheet of paper.
2. Description of the Related Art
A fusing device for fusing a toner image on a sheet of paper is used for an image forming apparatus such as a copying machine, a facsimile machine and the like. The fusing device includes a thermal roller having a heating means like a heater and a press roller for pressing against the thermal roller, and the toner image is thermally fused on the sheet of paper pressed between the thermal roller and press roller.
In such a fusing device, a thermistor for detecting a surface temperature of the thermal roller is attached to a housing enclosing the thermal roller, and temperature data obtained by the thermistor is applied to a temperature regulating device for regulating a heater provided in the thermal roller.
3. Disadvantages
Temperature control to a fusing device must be highly accurate because it is influential in fusibility of a toner image and occurrence of wrinkles in a sheet of paper. Although accurate temperature detection is needed, sometimes a temperature of a supplied sheet of paper is different from the atmospheric temperature within the image forming apparatus, especially, the temperature at the surface of the sheet of paper is different from the atmospheric temperature immediately after the sheet of paper is put in a sheet cassette, and in such a situation, the sheet of paper supplied to the fusing device is pressed against the thermal roller to change a temperature at the surface of a thermal roller. This results in fusibility of the toner image varying from one sheet to another.
A temperature sensor like a thermistor merely measures the temperature at the surface of the thermal roller in the fusing device or the atmospheric temperature within the fusing device. Humidity within the fusing device is one of major causes of occurrence of wrinkles in the sheet of paper in the fusing device. Thus, it is insufficient controlling the temperature at the fusing device based upon the temperature at the surface of the thermal roller or the atmospheric temperature within the fusing device in order to prevent the occurrence of wrinkles in the sheet of paper in the prior art image forming apparatus.